1. Field of the Invention
The present invention relates to a crystallization apparatus which irradiates laser light to a thin film such as a semiconductor film and to a crystallization method, particularly to a laser crystallization apparatus comprising means for correcting positional shift of a semiconductor film with respect to an imaging position of laser light, and a laser crystallization method.
2. Description of the Related Art
A laser crystallization technique for melting and crystallizing a non-single crystal semiconductor thin film using, for example, short pulse laser light having large energy is used to crystallize a semiconductor thin film used for manufacturing a thin film transistor for a liquid crystal display device, an organic electro luminescence display device or the like.
Among such laser crystallization technologies, attention is focused on a Phase Modulated Excimer Laser Annealing (PMELA) which uses an irradiation of a phase-modulated excimer laser light for crystallization. In the PMELA technique, excimer laser light having predetermined light intensity distribution whose phase has been modulated by a phase modulating element, for example, a phase shifter, is used. The excimer laser light is irradiated, for example, to a non-single crystal semiconductor thin film, for example, an amorphous silicon or polycrystal silicon thin film, formed on a glass substrate. The semiconductor film is molten at the irradiated portion then crystallized. In the presently developed PMELA technique, a region having a size of approximately several millimeter square is molten and crystallized by one irradiation, and a crystallized silicon thin film having comparatively uniform crystal grains of about several micrometers to 10 μm in size and having a superior quality has been formed (see, e.g., Kohki Inoue, Mitsuru Nakata, Masakiyo Matsumura, “Amplitude and Phase Modulated Excimer-Laser Melt-Regrowth Method of Silicon Thin-Films—A New Growth Method of 2-D Position-Controlled Large-Grains—”, Journal of the Society of Electron Information Communication, Vol. J85-C, No. 8, pp. 624 to 629, 2002).
In the PMELA technique, in order to obtain a crystallized semiconductor film having a stabilized quality, it is one of important technical problems to constantly align a position of the non-single crystal semiconductor thin film being crystallized to an imaging position of the excimer laser light in every laser light irradiation for crystallization. Since a substrate to be crystallized has a large area, for example, 550 mm×650 mm, the substrate has its own warp, thickness variation, deflection in installing to the PMELA apparatus and the like. Therefore, substantial flatness of the substrate is generally worse than a focal depth of a laser optical system, for example, ±5 μm to 10 μm.
To solve the problem, some conventional laser crystallization apparatus comprises a fixed substrate height measurement sensor. Examples of typical fixed substrate height measurement sensor include an optical system, electrostatic capacitance system, or gas pressure system. Substrate height correction using such fixed sensor is suitable for correcting a shift caused by the above-described flatness of the substrate.
However, in the laser crystallization apparatus, as described later in detail, since crystallization laser light having high energy is used, lens temperature of an excimer imaging optical system rises during use, and it causes a shift in the imaging position of the excimer imaging optical system. In a correction method using the conventional fixed substrate height measurement sensor, the shift of the imaging position caused by the laser crystallization apparatus itself due to the temperature change in the excimer imaging optical system cannot be corrected in principle.
As described above, in the laser crystallization apparatus, the imaging position of the crystallization laser light is preferably constantly aligned to the non-single crystal semiconductor thin film to be molten and crystallized in order to improve and stabilize the quality of the crystallized semiconductor film. If a shift is generated between the imaging position of the crystallization laser light and the position (height) of the non-single crystal semiconductor thin film disposed on the substrate, then desired crystallization is not performed in a crystallization step after melting the semiconductor thin film.
Especially in the PMELA apparatus, the crystallization laser light to be irradiated to the non-single crystal semiconductor thin film is modulated by an optical system in such a manner as to have a predetermined light intensity distribution on the non-single crystal semiconductor thin film being crystallized. However, if the shift is caused between the imaging position of the laser light and the position of the non-single crystal semiconductor thin film, then a predetermined light intensity distribution cannot be obtained on the non-single crystal semiconductor thin film. Therefore, a micro temperature distribution in the laser light irradiation region changes from a predetermined distribution. As a result, a desired melting or crystallization of semiconductor film is not performed. Specifically, a size of crystal grain grown is reduced. Additionally, the sizes of the crystal grains become nonuniform, thus the quality of the crystallized silicon film is degraded.
As the excimer laser light used in the PMELA apparatus, for example, krypton fluoride (KrF) or xenon chloride (XeCl) is preferred, and they have wavelengths of 248 nm and 308 nm, respectively. In the PMELA apparatus, the excimer laser light having a predetermined light intensity distribution formed by an optical phase modulating element such as a phase shifter is irradiated onto the non-single crystal semiconductor thin film. The excimer laser light should be imaged on the non-single crystal semiconductor thin film with a high resolution of about several μm.
As to the excimer laser light, the PMELA apparatus uses the laser light at a high light intensity, at high duty, and in a large area for production efficiency. The light preferably has a high intensity of about 1 J/cm2 on the semiconductor thin film to be crystallized. This is much larger than an intensity of an excimer laser light used in an aligner for semiconductor integrated circuit production. In order to obtain the high light intensity, in the PMELA apparatus, the excimer laser light is used with a wide spectral bandwidth (0.5 nm) without narrowing the bandwidth, unlike in an aligner used for large-scale integrated circuit production.
Moreover, there are limited lens materials capable of dealing with the excimer laser light which is ultraviolet light used in the PMELA apparatus, thus fused silica or synthetic quartz (referred as fused silica hereinafter) for ultraviolet or calcium fluoride (CaF2) is preferable from absorption characteristic and the like of the light. As described above, in the PMELA apparatus using the phase shifter, chromatic aberration correction of a lens is important since the excimer laser light having a large spectrum width is imaged in a high resolution on the non-single crystal semiconductor thin film. However, a constitution of an optical system including a laminated lens like a microscope lens for visible light is not applicable from the respect of heat resistance. Therefore, since the chromatic aberration has to be corrected with the above-described limited lens material, the number of the lenses increases.
If the excimer laser light having the high intensity is used in such lens system, although the excimer laser light is absorbed a little in each lens constituting the optical system, then total laser light absorption in the optical system is large. As a result, a so-called thermal lens effect is caused, for example, lens temperature rises, or the lens is strained. That is, a problem occurs that the imaging position shifts by the thermal lens effect. In one PMELA apparatus, the imaging position shifts, for example, by 10 μm, when the lens temperature changes by 1° C. in the excimer laser imaging optical system. Considering that the focal depth of the imaging optical system of the PMELA apparatus is about ±10 μm, the shift of the imaging position caused by the temperature change is not negligible.
The imaging position shift causes a variation in the quality of the crystallized semiconductor film such as a substrate for a liquid crystal display device having a large area, if the pulse laser light is repeatedly irradiated. As a result, there occur problems such as variation in image switching characteristic of the element formed therein, or a strained image.
In the conventional fixed substrate height correction system, a height shift of the substrate to be crystallized due to the flatness of the substrate itself can be corrected, as described above. However, correction for, for example, the shift of the imaging position caused by above-described thermal lens effect in optical system for excimer laser crystallization has not been considered.
Therefore, there is a need for a laser crystallization apparatus and a laser crystallization method in which a quality of a crystallized semiconductor thin film is prevented from being degraded and in which sizes of crystal grains are uniformed.